mailbox: mtk-cmdq: Fix uninitialized variable in cmdq_mbox_flush()

The "cb" pointer needs to be initialized before can assign
"data.data = cb->data;".

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>